Transit diversity wireless communication

ABSTRACT

A method of transmitting data from a transmitter ( 13,24 ) to a remote receiver ( 14,28 ) using transmit diversity wireless communication, the transmitter ( 13,24 ) comprising three or more transmit antenna elements ( 1,2,3,4 ). The data is encoded in symbol blocks ( 12 ), the symbols (s 1 ,s 2 ,s 3 ,s 4 ) of a block ( 12 ) being permuted within respective sub-sets of symbols between the transmit antenna elements ( 1,2,3,4 ) over time with respective replications and complex conjugations and/or negations. At least one of the sub-sets of said transmit antenna elements ( 1,2,3,4 ). The signals transmitted over at least one of the transmit antenna elements ( 1,2,3,4 ) are modified as a function of channel information at least approximately related to the channel transfer function (h 1 ,h 2 ,h 3  and h 4 ) of the transmitted signals, and the sub-sets of symbols and permuted symbols are permuted over time between said sub-sets of transmit antenna elements, so that the received signal is detectable at the receiver ( 14,28 ) using an orthogonal detection matrix scheme.

This application claims the benefit of prior filed co-pendinginternational application Serial No. PCT/EP02/08423 filed Jul. 29, 2002,and assigned to Motorola, Inc., which was published by the InternationalBureau on Feb. 27, 2003 under No. WO 03/017528 A1 and European PatentConvention Application No. 01402162.0 filed Aug. 13, 2001.

FIELD OF THE INVENTION

This invention relates to transmission of data by transmit diversitywireless communication.

BACKGROUND OF THE INVENTION

Wireless communication systems are assuming ever-increasing importancefor the transmission of data, which is to be understood in its largestsense as covering speech or other sounds and images, for example, aswell as abstract digital signals.

Currently proposed standards for wireless communication systems betweena stationary base station and a number of remote (mobile or immobile)stations include the 3GPP (3^(rd) generation Partnership Project) and3GPP2 standards, which use Frequency Division Duplex (‘FDD’) or TimeDivision Duplex (‘TDD’) and Code Division Multiple Access (‘CDMA’). TheHIPERLAN and HIPERLAN2 local area network standards of the EuropeanTelecommunications Standards Institute (‘ETSI’), use Time DivisionDuplex (‘TDD’) and Orthogonal Frequency Division Multiplex (‘OFDM’). TheInternational Telecommunications Union (‘ITU’) IMT-2000 standards alsouse various multiplex techniques of these kinds. The present inventionis applicable to systems of these kinds and other wireless communicationsystems.

In order to improve the communication capacity of the systems whilereducing the sensitivity of the systems to noise and interference andlimiting the power of the transmissions, various techniques are usedseparately or in combination, including space diversity, where the samedata is transmitted over different physical paths interleaved in time,in particular over different transmit and/or receive antenna elements,and frequency spreading where the same data is spread over differentchannels distinguished by their sub-carrier frequency.

At the receiver, the detection of the symbols is performed utilisingknowledge of the complex channel attenuation and phase shifts: theChannel State Information (‘CSI’). The Channel State Information isobtained at the receiver by measuring the value of pilot signalstransmitted together with the data from the transmitter. The knowledgeof the channel enables the received signals to be processed jointlyaccording to the Maximum Ratio Combining technique, in which thereceived signal is multiplied by the Hermitian transpose of theestimated channel transfer matrix.

Two broad ways of managing the transmit diversity have been categorisedas ‘closed loop’ and ‘open loop’.

Two closed loop methods are described in the paper entitled “Transmitadaptive array without user-specific pilot for 3G CDMA” by B.Raghothaman et al., that appeared in the IEEE Transactions 2000. In thesystems described in this paper, the signals transmitted over thedifferent transmit antenna elements of the base station are weightedaccording to relative weights calculated at the receiver from ChannelState Information and retransmitted to the transmitter. In one systemreferred to, pilots specific to each user are transmitted in addition tothe pilots for each transmit antenna element that are common to allusers, which penalises the communication capacity of the system. Inanother system disclosed in the paper, user-specific pilots are avoidedby re-modulating the detected signals using the measured Channel StateInformation and the calculated weights and using the re-modulatedsignals to correct errors in feedback; this imposes a heavycomputational load on the receiver and the result is only reliable ifthe channel state estimation is sufficiently correlated with the actualchannel state to avoid a high detection error rate.

In pure ‘open loop’ methods, no Channel State Information is fed back tothe transmitter. In such systems, the transmitter comprises a pluralityof transmit antenna elements; the data is encoded in symbol blocks, thesymbols of a block being permuted between the transmit antenna elementsover time with respective replications and complex conjugations and/ornegations. The complexity of the receiver depends on the properties ofthe matrix that defines this space-time block code; in particulardetection is performed with a low cost in terms of simplicity of thereceiver computations if this matrix is an orthogonal one. Orthogonalmatrices are well known: definitions are given in textbooks such as‘Matrix Computations’ by Gene H. Golub and Charles F. Van Loan, 3^(rd)Edition, published by Johns Hopkins. See page 69 (for a set of vectors)or page 208 (for a matrix).

An open loop system using an orthogonal detection matrix is described inInternational Patent Application Publication No WO 99/14871 Alamouti. Inthis system, the symbols of a block transmitted are permuted between thetransmit antenna elements over time with respective replications andcomplex conjugations and/or negations according to a scheme, known asthe ‘Alamouti code’, such that the received signal is detectable at thereceiver using an orthogonal detection matrix scheme.

The performance of the code is mainly based on the diversity order ofthe code. This diversity order characterizes the number of transmit andreceive antennas which is actually seen by the code. For a given numberof receive antenna elements, the more transmit antenna elements are usedthe more improvement is obtained in terms of fading and interference isobtained. However, the paper entitled “Space-Time Block Codes fromOrthogonal Designs” by V. Tarokh et al. that appeared in IEEETransactions on IT, vol. 45, Jul. 1999, states that an orthogonaldetection code matrix can not be used if the transmitter comprises morethan two transmit antenna elements with full diversity withoutsacrificing the coding rate, that is to say the useful data rate for theuser. They propose coding rates of ½ for three to eight transmit antennaelements or ¾ for three or four transmit antenna elements.

Patent specification WO 00/51265, Whinnett et al., assigned to Motorola,describes another transmit diversity system, in which code rate ismaintained for arrays of more than two transmit antenna elements but atthe expense of sub-optimal transmit diversity.

Another transmit diversity scheme (ABBA code) is described for more thantwo transmit antenna elements in the paper entitled “MinimalNon-Orthogonality Rate 1 Space-time Block Code for 3+Tx Antennas” by O.Tirkkonen et al. IEEE 6^(th) Int. Symp. On Spread-Spectrum Tech. &Appli., NJIT, pp. 429-432, September 2000. This coding rate 1 scheme isderived from the permutation of two Alamouti codes as described by thecode matrix

$\quad\begin{bmatrix}A & B \\B & A\end{bmatrix}$It is stated that the ABEA code provides full spatial diversity to thedetriment of the orthogonality of the detection matrix, which impliesthat the computational cost of the detection step is increased comparedto an orthogonal scheme. In addition the performance of the ideal codeis not fully achieved by the ABBA code due to the interference terms ofthe detection matrix.

Other compromises are proposed in a paper presented by H. Jafarkhani tothe IEEE Wireless Communications and Networking Conference in September2000 with non-orthogonal detection matrices that are stated not toachieve simultaneously the optimum diversity and transmission rate, twoencoding schemes proposed being of the kind described by the codematrices

$\begin{bmatrix}A & B \\B^{*} & {- A^{*}}\end{bmatrix}\mspace{14mu}{{{and}\mspace{14mu}\begin{bmatrix}A & B \\{- B^{*}} & A^{*}\end{bmatrix}}.}$

Yet another compromise is described in the paper “A randomizationtechnique for non-orthogonal space-time code blocks” by A Hottinen etal. appearing in IEEE VTC 2001. However, this system still does notemploy an orthogonal detection matrix with full diversity for more thantwo transmit antenna elements.

Still another compromise is described in the paper “A space-time codingapproach for systems employing four transmit antennas” by C. B. Papadiaset al. presented at an IEEE conference in 2001 and that proposes anencoding scheme of the kind

$\begin{bmatrix}b_{1} & b_{2}^{*} & b_{3} & b_{4}^{*} \\b_{2} & {- b_{1}^{*}} & {- b_{4}} & b_{3}^{*} \\b_{3} & b_{4}^{*} & {- b_{1}} & {- b_{2}^{*}} \\b_{4} & {- b_{3}^{*}} & b_{2} & {- b_{1}^{*}}\end{bmatrix}.$This scheme also uses a non-orthogonal detection matrix that does notachieve simultaneously the optimum diversity and transmission rate.

SUMMARY OF THE INVENTION

The present invention provides a method of transmitting data from atransmitter to a remote receiver using transmit diversity wirelesscommunication and a system, a transmitter and a receiver as claimed inthe accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system for transmitting data bytransmit diversity wireless communication in accordance with anembodiment of the invention,

FIG. 2 is a schematic diagram of a system in accordance with FIG. 1applied to a time division duplex (TDD), orthogonal frequency divisionmultiplex (OFDM) system, and

FIG. 3 is a schematic diagram of a system in accordance with FIG. 1applied to a frequency division duplex (FDD), code division multipleaccess (CDMA) system

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a first embodiment of a system for transmitting data by atransmit diversity wireless communication network, the system comprisinga first station that will be described as the transmitter side (withprimary reference to its transmission function) and a second stationthat will be described as the receiver side (with primary reference toits reception function). In the present case, the first station and thesecond station are both capable of both transmission and reception and,moreover, the same antenna elements are used both for transmission andreception in the preferred embodiment of the invention.

The transmitter side comprises four transmit antenna elements, 1, 2, 3,4. The receiver side of the system comprises an array 5 of M receiveantenna elements. The number of antenna elements 5 on the receiver sideis chosen on the basis of economical considerations to provide increasedchannel diversity; in the case of mobile telephony, a single basestation serves many hundreds or even thousands of mobile units and it istherefore more economical to add antenna elements to the base stationthan to the mobile units. In the case of a local area network (‘LAN’),for example, the cost of the remote stations is less critical and ahigher number of antennas will be chosen on the receiver side.

Each transmit antenna element 1 to 4 transmits over a variety of pathsto each of the receive antenna elements 5. Thus, considering the m^(th)receive antenna element out of a total of M, each of the transmitantenna elements 1 to 4 transmits to the receive antenna element m overa variety of paths due to multiple reflections and scattering, whichintroduce complex multi-path fading; however, for simplification, theprocessing of the signals at the receiver is described and illustratedas if they were subject to flat fading (equivalent to transmission overa single path with no inter-path interference) that can be representedby a complex channel transfer coefficient h_(1m) to h_(4m).

In operation, symbols s₁, s₂, s₃, s₄ are derived from the data to betransmitted and applied to the transmit antennas 1, 2, 3 and 4. Thereceiver side of the system comprises a detector 6 which receivessignals from the receive antenna element array 5 and detects the symbolss₁ to s₄ from the receive antenna elements.

On the transmit side of the system, a channel state information unitextracts weights w₁, w₂, w₃ and w₄ that are, in general terms, a complexfunction of the channel transfer coefficients h₁, h₂, h₃, and h₄ foreach of the transmit antenna elements 1, 2, 3, 4. Before transmission,the signal to be transmitted from each of the antenna elements 1 to 4 ismultiplied by the respective weight w₁ to w₄. The weight is again acomplex coefficient, which is a function of the transfer channelcoefficient and hence the signal may be modified in phase and/oramplitude as a function of the channel state information.

The data symbols to be transmitted are encoded in symbol blocks and thesymbols are permuted over time within each block between the transmitantenna elements 1 to 4 with respective replications and complexconjugations and/or negations, so that the received signal is detectableat the receiver side using an orthogonal detection matrix scheme. Theencoding scheme matrix for the symbol blocks is shown at 8 in FIG. 1.

The symbols s₁ to s₄ are permuted over the transmit antenna elements anumber of times which is a power of 2, the power being greater than orequal to 2, the block comprising four permutations in the present case.It is also possible for the transmitter to include three antennaelements, the block of symbols preferably comprising four permutationsin this case also. A higher number of permutations may also be utilisedbut will prolong the symbol block transmission.

As shown in FIG. 1, the symbols within each block are per muted in pairswithin respective subsets of the symbols and transmitted overcorresponding subsets of the transmit antenna elements 1 to 4, thesubsets of symbols subsequently being permuted between the subsets ofthe transmit antenna elements. Thus, as shown in FIG. 1, symbols s₁, s₂are transmitted initially over transmit antenna elements 1, 2 andsymbols s₃, s₄ are transmitted initially over transmit antenna elements3, 4. In the next step, the negation and conjugation of the symbol s₂ istransmitted over the transmit antenna element 1 and the conjugation ofthe symbol s₁ is transmitted over the transmit antenna element 2, thenegation and conjugation of the symbol s₄ being transmitted over thetransmit antenna element 3 and the conjugation of the symbol s₃ beingtransmitted over the transmit antenna element 4. It will be understoodthat the symbols s₁ and s₂ and their negations and/or their conjugationsconstitute a first subset of symbols that is transmitted over the subsetof transmit antenna elements 1 and 2 with permutations and the symbolss₃ and s₄ with their negations and/or conjugations are transmitted overthe subset of transmit antenna elements 3 and 4 with permutations. Inthe next step, the subset including symbols s₃ and s₄ is transmittedover the subset of transmit antenna elements 1 and 2 with permutationswhile the subset of symbols s₁, s₂ is transmitted over the subset oftransmit antenna elements 3 and 4 with permutations.

This encoding scheme is a scheme of the kind ABBA. This embodiment ofthe present invention enables this encoding scheme to be decoded by anorthogonal detection matrix scheme at the receiver. It is also possiblefor other encoding schemes of analogous nature to be decoded using anorthogonal detection matrix scheme, for instance ABB*-A*, or AB-B*A*.Moreover, the space-time code has an overall coding rate of one (that isto say that the data rate is as high as in a single antenna case) andthe system derives full benefit from the spatial diversity of themultiple transmit and receive antenna elements at the transmitter andreceiver. The fact that the detection scheme uses an orthogonal matrixenables the detection to be performed with low computational cost.Interference terms that would be present with a non-orthogonal detectionmatrix scheme are substantially cancelled out by the application of theweights w₁ to w₄ to the signals transmitted as a function of theestimated channel transfer functions.

The signal Y_(m) received by the m^(th) antenna over four time instantswithin the symbol block can be written as

$\begin{matrix}{\underset{\underset{Y_{m}}{︸}}{\begin{bmatrix}\begin{matrix}\begin{matrix}y_{m,1} \\y_{m,2}^{*}\end{matrix} \\y_{m,3}\end{matrix} \\y_{m,4}^{*}\end{bmatrix}} = {{\underset{\underset{H_{m}}{︸}}{\begin{bmatrix}{h_{1m}w_{1}} & {h_{2m}w_{2}} & {h_{3m}w_{3}} & {h_{4m}w_{4}} \\{h_{2m}^{*}w_{2}^{*}} & {{- h_{1m}^{*}}w_{1}^{*}} & {h_{4m}^{*}w_{4}^{*}} & {{- h_{3m}^{*}}w_{3}^{*}} \\{h_{3m}w_{3}} & {h_{4m}w_{4}} & {h_{1m}w_{1}} & {h_{2m}w_{2}} \\{h_{4m}^{*}w_{4}^{*}} & {{- h_{3m}^{*}}w_{3}^{*}} & {h_{2m}^{*}w_{2}^{*}} & {{- h_{1m}^{*}}w_{1}^{*}}\end{bmatrix}}\mspace{11mu}\underset{\underset{S}{︸}}{\begin{bmatrix}\begin{matrix}\begin{matrix}s_{1} \\s_{2}\end{matrix} \\s_{3}\end{matrix} \\s_{4}\end{bmatrix}}} + \underset{\underset{B_{m}}{︸}}{\begin{bmatrix}\begin{matrix}\begin{matrix}b_{m,1} \\b_{m,2}^{*}\end{matrix} \\b_{m,3}\end{matrix} \\b_{m,4}^{*}\end{bmatrix}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where y_(m,1) to y_(m,4) represent the signals received from thetransmit antenna elements 1 to 4 respectively, H_(m) represents thematrix obtained by multiplying the channel transfer functions by thecorresponding weights applied to the transmit antenna elements, Srepresents the symbols s₁ to s₄ transmitted from the transmit antennaelements 1 to 4 respectively, b_(m,1) to b_(m,4) represent the noise andinterference at the m^(th) receive antenna element and B_(m) representsthe received noise matrix. In this equation, the minus sign representsthe negation of the corresponding value and the asterisk sign representsthe conjugate of the value.

These multiple received signals are processed at the receiver accordingto the Maximum Ratio Combining technique. That is to say, the receivedpilot signals for each transmit antenna element are measured in order toestimate the channel transfer coefficients h_(1m) to h_(4m) and theweights applied at the transmitter side w₁ to w₄ and the Hermitiantransposes Ĥ_(m) ^(H) of the estimated channel transfer coefficientmatrices for each receive antenna element m are calculated. The receivedsymbol blocks Y_(m) are multiplied by the corresponding Hermitiantransposes Ĥ_(m) ^(H) and the resulting multiplied signals are summedover the antennas, and we finally get a new signal Z such that

$\begin{matrix}{Z = {{\sum\limits_{m = 1}^{M}{{\hat{H}}_{m}^{H}Y_{m}}} = {{\left( {\sum\limits_{m = 1}^{M}{{\hat{H}}_{m}^{H}H_{m}}} \right)S} + {\sum\limits_{m = 1}^{M}{{\hat{H}}_{m}^{H}{B_{m}.}}}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Provided that the channel estimation is sufficiently accurate and theweights actually applied to the signals to be transmitted alsocorrespond accurately to the calculated weights, the resulting detectionmatrix

$\sum\limits_{m = 1}^{M}{{\hat{H}}_{m}^{H}H_{m}}$corresponds with a sufficient degree of approximation to the detectionmatrix of the ideal orthogonal rate 1 code scheme for proper detectionof the data, that is to say

$\begin{matrix}{{{\sum\limits_{m = 1}^{M}{H_{m}^{H}H_{m}}} = {{\begin{bmatrix}A & 0 & 0 & 0 \\0 & A & 0 & 0 \\0 & 0 & A & 0 \\0 & 0 & 0 & A\end{bmatrix}\mspace{14mu}{where}\mspace{14mu} A} = {\sum\limits_{m = 1}^{M}{\sum\limits_{n = 1}^{4}{h_{nm}}^{2}}}}},} & {{Equation}\mspace{14mu} 3}\end{matrix}$if the transmit weights satisfy at least approximately the followingrelation:

$\begin{matrix}{{\Re\left\{ {{\sum\limits_{m = 1}^{M}{h_{1m}^{*}h_{3m}w_{1}^{*}w_{3}}} + {\sum\limits_{m = 1}^{M}{h_{2m}^{*}h_{4m}w_{2}^{*}w_{4}}}} \right\}} = 0.} & {{Equation}\mspace{14mu} 4}\end{matrix}$where h_(nm) is the channel transfer coefficient of the channel betweenthe n^(th) transmit antenna element and the m^(th) receive antennaelement, w_(n) is the weight applied to the signal of the n^(th)transmit antenna element and

represents the real part of the value on which it operates.

Due to the orthogonality of the code scheme, the detection step can thenbe performed with a low computational cost.

Several sets of transmit weights may be used to solve this equation inaccordance with this embodiment of the present invention. One exampleconsists in choosing the four weights such that

$\begin{matrix}{{w_{1} = 1}{w_{2} = 1}{w_{3} = {\exp\left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{h_{1m}h_{3m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}}{w_{4} = {{\exp\left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{h_{2m}h_{4m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}.}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

With this choice of weights, it is sufficient for the transmitting sideto obtain phase information for the weighting operation on two out ofthe four transmit antenna elements, which reduces the amount of feedbackinformation to be transmitted from the receiver side if the channelstate information is measured at the receiver side, for example.

The detection scheme for three emitting antennas can easily be derivedfrom this four antenna coding scheme. Four complex symbols aretransmitted from three antennas over four time instants, for instance byturning off the 4^(th) antenna, which corresponds to set h_(4m)=0 in theprevious equations. In this case the overall space-time scheme is anorthogonal one if and only if the transmit weights satisfy:

$\begin{matrix}{{{\Re\left\{ {\sum\limits_{m = 1}^{M}{h_{1m}^{*}h_{3m}w_{1}^{*}w_{3}}} \right\}} = 0},} & {{Equation}\mspace{14mu} 6}\end{matrix}$for instance by choosing

$\begin{matrix}{w_{1} = {w_{2} = {{1\mspace{14mu}{and}\mspace{14mu} w_{3}} = {{\exp\left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{h_{1m}h_{3m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}.}}}} & {{Equation}\mspace{14mu} 7}\end{matrix}$

With this choice of weights, it is sufficient for the transmitting sideto obtain phase information the weighting operation on one only out ofthe three transmit antenna elements.

The above conditions for the weighting scheme to enable decoding by anorthogonal detection matrix are applicable to an encoding scheme of thekind ‘ABBA’, that is to say where sub-sets A and B of symbols s₁, s₂ ands₃, s₄ and conjugated symbols s₁*, s₂* and s₃*, s₄* and/or negatedconjugated symbols −s₁*, −s₂* and −s₃*, −s₄* symbols are permuted overtime between sub-sets 1, 2 and 3, 4 of antenna elements without negationnor conjugation of the symbol sub-sets, according to the matrix

$\begin{bmatrix}A & B \\B & A\end{bmatrix}.$

Other encoding schemes may be utilised of the form

$\begin{bmatrix}A & B \\B^{*} & {- A^{*}}\end{bmatrix}\mspace{14mu}{{or}\mspace{14mu}\begin{bmatrix}A & B \\{- B^{*}} & A^{*}\end{bmatrix}}\mspace{14mu}{{{or}\mspace{14mu}\begin{bmatrix}b_{1} & b_{2}^{*} & b_{3} & b_{4}^{*} \\b_{2} & {- b_{1}^{*}} & {- b_{4}} & b_{3}^{*} \\b_{3} & b_{4}^{*} & {- b_{1}} & {- b_{2}^{*}} \\b_{4} & {- b_{3}^{*}} & b_{2} & {- b_{1}^{*}}\end{bmatrix}}.}$In the absence of weighting before transmission, these encoding schemeswould leave interference terms that would require a detection matrix

$\sum\limits_{m = 1}^{M}{H_{m}^{H}H_{m}}$that is non-orthogonal.In order to be able to detect the signals using an orthogonal detectionmatrix, in accordance with another embodiment of the present inventionthe weighting applied to the signals to be transmitted are derived suchthat

$\begin{matrix}{{\Re\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0.} & {{Equation}\mspace{14mu} 8}\end{matrix}$

In a preferred realization of this embodiment,

$\begin{matrix}\left\{ \begin{matrix}{w_{p} = {w_{q} = 1}} \\{w_{r} = {\exp\left( {j \times \left\lbrack {{{angle}\left\{ {\sum\limits_{m = 1}^{M}{h_{pm}h_{rm}^{*}}} \right\}} + {\pi/2}} \right\rbrack} \right)}} \\{w_{s} = {\exp\left( {j \times \left\lbrack {{{angle}\left\{ {\sum\limits_{m = 1}^{M}{h_{qm}h_{sm}^{*}}} \right\}} + {\pi/2}} \right\rbrack} \right)}}\end{matrix} \right. & {{Equation}\mspace{14mu} 9}\end{matrix}$

In accordance with yet other embodiments of the present invention, inequation 8,

$\begin{matrix}{{{??}\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0} & {{Equation}\mspace{14mu} 10}\end{matrix}$where

represents the imaginary part of the value it operates and the scheme isdecodable using an orthogonal detection matrix.

Preferably,

$\begin{matrix}\left\{ \begin{matrix}{w_{p} = {w_{q} = 1}} \\{w_{r} = {\exp\left( {j \times {angle}\left\{ {\sum\limits_{m = 1}^{M}{h_{pm}h_{rm}^{*}}} \right\}} \right)}} \\{w_{s} = {\exp\left( {j \times {angle}\left\{ {\sum\limits_{m = 1}^{M}{h_{qm}h_{sm}^{*}}} \right\}} \right)}}\end{matrix} \right. & {{Equation}\mspace{14mu} 11}\end{matrix}$

FIG. 2 shows the application of a system in accordance with FIG. 1 to atime division duplex (TDD) system based on orthogonal frequency divisionmultiplexing (OFDM) modulation, as specified for example in theHiperlan/2 standard of the ETSI but modified to include the space-timetransmit diversity system of the embodiment of the present inventionshown in FIG. 1. The embodiment of the invention shown in FIG. 2 hasfour transmit aerials at the base station, but it is also possible forthe base station to have three transmit aerials. The base station of thesystem is shown at 13 and one of a number of subscriber units is shownat 14.

At the base station 13, data is input to the encoder 15 where it isencoded to include error correction information. The resulting datatrain is supplied to a data-mapping and block coding unit that forms thedata train into symbols and performs the negation and conjugationoperations to produce the symbol blocks according to the encoding scheme12. The data from the mapping and block coding unit 16 is supplied tothe multipliers 8, 9, 10 and 11 where, for each sub-carrier of frequencyf, it is multiplied by respective complex weighting coefficients w_(1f)to w_(4f) and applied to respective elements of an array of OFDMmodulation units that feed the transmit antennas 1 to 4. It is alsopossible to apply interleaving of the data after the encoder 15.

Pilot signals are included in the transmitted signals for each transmitantenna element, without weighting, to enable estimation of the downlinkchannels. A permutation signal is also added that is indicative of thenumber of permutations in a symbol block. For example, especially duringdeployment of the present invention, there may be a mix of base stationswith only two transmit antenna elements and therefore two permutationsper symbol block and base stations in accordance with the presentinvention with more than two transmit antenna elements and thereforemore than two permutations per symbol block. The permutation signaltakes a distinctive value, at least in the latter case, to enable thereceiver to adapt the number of permutations performed in detecting thesignal to the number made in the transmitted signal.

The channel state information is calculated in the calculator 7 at thebase station from a similar pilot signal included in the uplinktransmissions from the respective subscriber unit 14 and received at thebase station over the antenna elements 1, 2, 3 and 4 and detected by thereceiver unit 14 of the base station; since the system is a timedivision duplex system with the same carrier frequencies used for thedownlink and the uplink, the measurements made on the uplink pilot areconsidered to be a sufficient approximation to the state of the downlinksignal.

At the subscriber unit 14, the transmitted signals are received over thearray of receive antenna elements 5, which are also used fortransmission of signals back to the base station. The received signalsare demodulated in respective OFDM channel demodulators 18. The channeltransfer coefficients are estimated by an array of channel estimators 19from the downlink pilot signal and applied to respective receiverelements of an array 20, together with the permutation signal thatindicates the number of permutations to be performed in detecting thesymbols. The receiver elements of array 20 calculates the Hermitiantransform Ĥ_(m) ^(H) of the channel transfer matrix, which it uses tomultiply the received signals in the array of receivers 20.

The processed signals from the array of receivers 20 are applied to themaximum ratio combination summer 21 that adds the signals from thereceiver array 20 over the antenna elements 5. Because of the transmitweights w₁ to w₄ applied at the transmitter, the detection matrix schemeis an orthogonal matrix. The signal from the maximum ratio combiner 21is passed to a matrix computation unit 22 that recovers the digitalsignal train from the symbol blocks. The digital signal train is passedto a decoder 23 that applies the error correction process and recoversthe data.

The system described with reference to FIG. 2 is a time division duplexsystem utilising orthogonal frequency division multiplexing. Thisenables the weighting of the transmit channels to be calculated at thebase station using measurement of a pilot signal in the uplink signaltransmitted from the subscriber station as an approximation for thechannel state information of the downlink signal transmitted from thebase station. Since the same antenna elements both at the base stationand at the subscriber station are used for reception and transmission,this approximation is valid, and, indeed, the approximation is alsovalid in certain circumstances even where the antenna elements used fortransmission and reception are not identical for the uplink anddownlink.

The system shown in FIG. 3 is a frequency division duplex system basedon the CDMA (code division multiple access) standards with amodification to provide some feedback information from the mobile unitsto the base station to provide some channel state information concerningthe downlink signal. Such a system is compatible with the 3GPP or 3GPP2standard if an adaptation to the standard were introduced to accommodatethe channel state information fed back from the mobile unit to the basestation.

Referring now to FIG. 3 in more detail, the base station comprises atransmitter part shown generally at 24 and a receiver part 25. Inputdata, together with a pilot signal and a permutation signal indicativeof the number of permutations made during the space-time transmitdiversity permutations is applied to the encoder 15. The encoder 15includes error correction data and the resulting signal is applied tothe data mapping and block coding unit 16 that assembles the train ofdigital data into symbol blocks with permutations, negations andconjugations according to the encoding scheme. Multipliers 8 to 11 thenmultiply the data signals by respective weights for the respectiveantenna elements 1, 2, 3 and 4. In accordance with the CDMAspecifications, the signals are spread over different frequencysub-carrier bands before transmission by an array of spreaders 26.

In the present embodiment of the invention, the channel stateinformation is calculated at the mobile unit and transmitted back to thebase station on the uplink over the same antenna elements as used forthe downlink. In the preferred embodiment, the parameters as inequations 9 or 10 for the weights w₁ to w₄ to be applied to themultipliers 8 to 11 are calculated at the mobile unit and transmitted onthe uplink to the base station, as this reduces the amount of feedbackinformation passing over the communication link, being only a phaseinformation for the weights w₃ and w₄ (w₃ only in the case of threetransmit antenna elements), the weights w₁ and w₂ being constant values.The signals received at the base station antenna elements 1 to 4 aredecoded in the receiver part 25 of the base station. The weightinginformation signal is extracted by a detector 27 and supplied to thechannel state calculator 7 to calculate the weights applied to themultipliers 8 to 11.

The mobile unit comprises a receiver part indicated generally at 28 anda transmitter part 29. At the mobile unit, the signals transmitted arereceived on the antenna element array 5 and applied to a correspondingarray of despreaders 30 that supply the base band signals to the arrayof receiver channel transfer function estimators at the mobile unit andto an array of receivers 31, each “finger” or signal received over adifferent transmission path being detected separately and the fingersbeing reassembled. The channel state information is calculated from apilot signal transmitted by the base station. The channel stateinformation is supplied on one hand to the mobile unit transmitter part29 for retransmission to the base station in the uplink signal (which isat a different frequency from the downlink signal) and to the array ofreceiver elements 31.

The receiver elements multiply the signals from the despreaders array 30by the coefficients of the Hermitian transform Ĥ_(m) ^(H) of the channeltransfer matrix obtained by permutation, transposition negation andconjugation operations on the channel state information signals, thenumber of permutations being defined by the received permutation signal.The resulting signals from each receiver antenna element are then summedover all fingers in an array of maximum ratio combiners 32 and furthersummed in a maximum ratio combiner 21 over the different antennaelements. Once again, the application of the transmit weightscorresponding to equation 4 ensures that the detection matrix is anorthogonal matrix that enables the calculations to be greatlysimplified. The symbols from the maximum ratio combiner 21 are appliedto the matrix computation unit 22 and converted to a chain of digitalsignals and the decoder 23 detects and recovers the data with errordetection.

In a preferred embodiment of this type of system, the transmitter part29 of the mobile unit is similar to the transmitter part 24 of the basestation in operation, and the receiver part 25 of the base station issimilar to the receiver part 28 of the mobile unit. Adaptations are ofcourse made to the number of antenna elements in the array 5 at thesubscriber station. In this way, advantage is taken of the space-timetransmit diversity performance of the present invention on the uplinkfrom the subscriber unit to the base station as well as on the downlinkfrom the base station to the mobile unit.

In another embodiment of the invention, the number of antenna elementsapplied at the mobile unit is reduced, for example to two antennaelements, with a view to reducing to the cost of the mobile unit. Inthis case, the spatial diversity is, of course, reduced compared to asystem with four transmit antennas in the array 5.

1. A method of transmitting data from a transmitter to a remote receiverusing transmit diversity wireless communication, said transmittercomprising at least three transmit antenna, elements, the methodcomprising the step of encoding said data in symbol blocks, whichincludes modifying signals to be transmitted over at least one of saidtransmit antenna elements as a function of channel information at leastapproximately related to the channel transfer function of thetransmitted signals (h₁, h₂, h ₃, and h₄), such that the received signalcan be detected at the receiver by a detection scheme using anorthogonal detection code matrix, the signals to be transmitted over atleast one of said transmit antenna elements are modified so as tosatisfy at least approximately the equation${\Re\left\{ {{\sum\limits_{m = 1}^{M}{h_{1m}^{*}h_{3m}w_{1}^{*}w_{3}}} + {\sum\limits_{m = 1}^{M}{h_{2m}^{*}h_{4m}w_{2}^{*}w_{4}}}} \right\}} = 0$or the equation${\Re\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0$where the complex number h_(nm) represents the actual channel transferfunction over the nth transmit antenna element and the math receivertransmit antenna element, the complex number w_(n) represents themodification applied to the signals at the nth transmit antennaelement, * represents the complex conjugate of the number associatedtherewith,

represents the real part of a complex value, and

represents the imaginary part of a complex value, and permuting thesymbols (s1, s2, s3, s4) of a block between the transmit antennaelements over time with respective replications and complex conjugationsand/or negations, permuting pairs of the symbols (s₁, s₂, s₃, s₄) withinsaid symbol blocks over time within respective sub-sets of symbols andpermuted symbols between the transmit antenna elements of respectivesub-sets of said transmit antenna elements, and permuting said sub-setsof symbols and permuted symbols over time between said sub-sets oftransmit antenna elements.
 2. A method as claimed in claim 1, whereinsaid signals transmitted ever said transmit antenna elements aremodified in phase as a function of said channel information.
 3. A methodas claimed in claim 1, wherein said channel information is a function ofsignals received at said receiver from said transmitter and istransmitted from said receiver to said transmitter.
 4. A method asclaimed in claim 1, wherein said receiver additionally comprisestransmitting means for transmitting signals over channels similar to thetransmit channels of said transmitter, and said transmitter additionallycomprises receiving means including said transmit antenna elements forreceiving signals transmitted from said transmitting means,characterized in that said channel information is calculated at saidtransmitter as a function of the channel transfer function of signalstransmitted from said transmitting means to said receiving means.
 5. Amethod as claimed in claim 1, wherein said symbols (s₁, s₂, s₃, s₄) arepermuted over time within said block between the transmit antennaelements a number of times equal to a power of two, which power isgreater than or equal to two.
 6. A method as claimed in claim 4, whereinsaid transmitter comprises three or four of said transmit antennaelements and said symbols (s₁, s₂, s₃, s₄) are permuted over time withinsaid block four times between the transmit antenna elements.
 7. A methodas claimed in claim 1, wherein said sub-sets of symbols and permutedsymbols are permuted over time between said sub-sets of transmit antennaelements without conjugation or negation and said signals transmittedover at least one of said transmit antenna elements are modified so asto satisfy at least approximately the equation${{??}\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0$where the complex numbers h_(1m), h_(2m), h_(3m) and h_(4m) representthe actual channel transfer functions over the transmit antenna elementsand the m^(th) receiver transmit antenna element, the complex numbersw₁, w₂, w₃ and w₄ represent the modifications applied to the signals atthe transmit antenna elements, x* represents the complex conjugate ofthe number x, and the operator

represents the real part of a complex value.
 8. A method as claimed inclaim 1, wherein said transmitter comprises four of said transmitantenna elements and said signals transmitted over said transmit antennaelements are modified so as to satisfy at least approximately theequations $\quad\begin{matrix}{w_{1} = 1} \\{w_{2} = 1} \\{w_{3} = {\exp\left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{h_{1m}h_{3m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}} \\{w_{4} = {\exp\left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{h_{2m}h_{4m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}}\end{matrix}$ where the complex number h_(nm) represents the measuredchannel transfer function over the nth transmit antenna element and themath receiver antenna element, M represents the total number of receiverantenna elements at said receiver, the complex number w_(n) representsthe modification applied to the signals at the nth transmit antennaelement, and x* represents the complex conjugate of the number x.
 9. Amethod as claimed in claim 8, wherein values at least approximatelyrelated to w₃ and w₄ are calculated at said receiver as a function ofreceived signals and transmitted from said receiver to said transmitter.10. A method as claimed in claim 8, wherein said receiver additionallycomprises transmitting means for transmitting signals over channelssimilar to the transmit channels of said transmitter, and saidtransmitter additionally comprises receiving means including saidtransmit antenna elements for receiving signals transmitted from saidtransmitting means, wherein values at least approximately related to w₃and w₄ are calculated at said transmitter as a function of the channeltransfer function (h₁, h₂, h₃ , and h₄) of signals transmitted from saidtransmitting means to said receiving means.
 11. A method as claimed inclaim 1, wherein said transmitter comprises three of said transmitantenna elements and said signals transmitted over said transmit antennaelements are modified so as to satisfy at least approximately theequations${w_{1} = {w_{2} = 1}},{{{and}\mspace{14mu} w_{3}} = {\exp\left( {j\left\lbrack {{{angle}\left( {\sum\limits_{m = 1}^{M}{h_{1m}h_{3m}^{*}}} \right)} + {\pi/2}} \right\rbrack} \right)}}$where the complex number h_(nm) represents the measured channel transferfunction over the nth transmit antenna element and the math receiverantenna element, M represents the total number of receiver antennaelements at said receiver, the complex number w_(n) represents themodification applied to the signals at the nth transmit antenna element,and x* represents the complex conjugate of the number x.
 12. A method asclaimed in claim 11, wherein a value at least approximately related tow₃ is calculated at said receiver as a function of received signals andtransmitted from said receiver to said transmitter.
 13. A method asclaimed in claim 11, wherein said receiver additionally comprisestransmitting means for transmitting signals over channels similar to thetransmit channels of said transmitter, and said transmitter additionallycomprises receiving means including said transmit antenna elements forreceiving signals transmitted from said transmitting means,characterized in that a value at least approximately related to w₃ iscalculated at said transmitter as a function of the channel transferfunction (h₁, h₂, h₃, and h₄) of signals transmitted from saidtransmitting means to said receiving means.
 14. A method as claimed inclaim 1, wherein a permutation signal indicative of the number ofpermutations over time of said symbols (s₁, s₂, s₃, s₄) within saidblock is transmitted from said transmitter to said receiver and saidreceiver is responsive to said permutation signal in the numbers ofpermutations it performs in detecting the data transmitted.
 15. A systemfor transmitting data by transmit diversity wireless communication, thesystem comprising a transmitter and a plurality of said remotereceivers, wherein said transmitter comprising at least three transmitantenna elements and transmit encoding means for encoding data in symbolblocks, said transmit encoding means being arranged for modifyingsignals transmitted over at least one of said transmit antenna elementsas a function of channel information at least approximately related tothe channel transfer function (h₁, h₂, h₃, and h₄) of the transmittedsignals, such that the received signal can be detected at the receiverby a detection scheme using an orthogonal detection code matrix, saidtransmit encoding means being arranged to modify the signals transmittedover at least one of said transmit antenna elements so as to satisfy atleast approximately the equation${\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0$or the equation${\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0$where the complex number h_(nm) represents the actual channel transferfunction over the nth transmit antenna element and the math receivertransmit antenna element, the complex number w_(n) represents themodification applied to the signals at the nth transmit antennaelement, * represents the complex conjugate of the number associatedtherewith,

represents the real part of a complex value, and

represents the imaginary part of a complex value, and the encoding meansbeing arranged to permute the symbols (s1, s2, s3, s4) of a blockbetween the transmit antenna elements over time with respectivereplications and complex conjugations and/or negations, permute pairs ofthe symbols within said symbol blocks over time within respectivesub-sets of symbols and permuted symbols between the transmit antennaelements of respective sub-sets of said transmit antenna elements, andsaid transmit encoding means is arranged to permute said sub-sets ofsymbols and permuted symbols over time between said sub-sets of transmitantenna elements.
 16. A system as claimed in claim 14, wherein saidreceiver comprises means for calculating said channel information as afunction of pilot signals received from said transmitter and fortransmitting said channel information from said receiver to saidtransmitter.
 17. A system as claimed in claim 14, wherein said receiveradditionally comprises transmitting means for transmitting signals overchannels similar to the transmit channels of said transmitter, and saidtransmitter additionally comprises receiving means including saidtransmit antenna elements for receiving signals transmitted from saidtransmitting means, wherein said transmit encoding means at saidtransmitter is responsive to the channel transfer function of pilotsignals transmitted from said transmitting means to said receiving meansto calculate said channel information.
 18. A system as claimed in claim15, wherein said encoding means is arranged to transmit a permutationsignal indicative of the number of permutations over time of saidsymbols within said block and said receiver comprises detection meansresponsive to said permutation signal in the number of permutations itperforms in detecting the data transmitted.
 19. A transmitter fortransmitting data to a remote receiver by transmit diversity wirelesscommunication, comprising at least three transmit antenna elements andtransmit encoding means for encoding data in symbol blocks, saidtransmit encoding means being arranged for modifying signals transmittedover at least one of said transmit antenna elements as a function ofchannel information at least approximately related to the channeltransfer function (h₁, h₂, h₃, and h₄) of the transmitted signals, suchthat a received signal can be detected at a receiver by a detectionscheme using an orthogonal detection code matrix, said transmit encodingmeans being arranged to modify the signals transmitted over at least oneof said transmit antenna elements so as to satisfy at leastapproximately the equation${\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0$or the equation${\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0$where the complex number h_(nm) represents the actual channel transferfunction over the nth transmit antenna element and the math receivertransmit antenna element, the complex number w_(n) represents themodification applied to the signals at the nth transmit antennaelement, * represents the complex conjugate of the number associatedtherewith,

represents the real part of a complex value, and

represents the imaginary part of a complex value, and the encoding meansbeing arranged to permute the symbols (s1, s2, s3, s4) of a blockbetween the transmit antenna elements over time with respectivereplications and complex conjugations and/or negations, permute pairs ofthe symbols within said symbol blocks over time within respectivesub-sets of symbols and permuted symbols between the transmit antennaelements of respective sub-sets of said transmit antenna elements, andsaid transmit encoding means is arranged to permute said sub-sets ofsymbols and permuted symbols over time between said sub-sets of transmitantenna elements.
 20. A transmitter as claimed in claim 19, fortransmitting data to a receiver additionally comprising transmittingmeans for transmitting signals over channels similar to the transmitchannels of said transmitter, wherein said transmitter additionallycomprises receiving means including said transmit antenna elements forreceiving signals transmitted from said transmitting means, saidtransmit encoding means at said transmitter being responsive to thechannel transfer function (h₁, h₂, h₃, and h₄) of signals transmittedfrom said transmitting means to said receiving means to calculate saidchannel information.
 21. A transmitter as claimed in claim 19, whereinsaid encoding means is arranged to transmit a permutation signalindicative of the number of permutations over time of said symbolswithin said block.
 22. A receiver for receiving data transmitted bytransmit diversity wireless communication from a transmitter comprisingat least three transmit antenna elements, said receiver comprisesdetection means for detecting data encoded in symbol blocks, fromsignals transmitted over at least one of said transmit antenna elementshaving been modified as a function of channel information at leastapproximately related to the channel transfer function (h₁, h₂, h₃, andh₄) of the transmitted signals such that a received signal can bedetected at the receiver by a detection scheme using an orthogonaldetection code matrix, said signals transmitted over at least one ofsaid transmit antenna elements having been modified so as to satisfy atleast approximately the equation${\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0$or the equation $\begin{matrix}{{\left\{ {{\sum\limits_{m = 1}^{M}{h_{pm}^{*}w_{p}^{*}h_{rm}w_{r}}} \pm {\sum\limits_{m = 1}^{M}{h_{qm}^{*}w_{q}^{*}h_{sm}w_{s}}}} \right\}} = 0} & \;\end{matrix}$ where the complex number h_(nm) represents the actualchannel transfer function over the nth transmit antenna element and themath receiver transmit antenna element, the complex number w_(n) ,represents the modification applied to the signals at the nth transmitantenna element, * represents the complex conjugate of the numberassociated therewith,

represents the real part of a complex value, and

represents the imaginary part of a complex value, and said detectionmeans is arranged to detect symbols (s1, s2, s3, s4) of a block permutedbetween the transmit antenna elements over time with respectivereplications and complex conjugations and/or negations, pairs of thesymbols within said symbol blocks permuted over time within respectivesub-sets of symbols and permuted symbols between the transmit antennaelements of respective sub-sets of said transmit antenna elements andsub-sets of symbols and permuted symbols permuted over time between saidsub-sets of transmit antenna elements.
 23. A receiver as claimed inclaim 22, wherein said detection means at said receiver comprises meansfor calculating said channel information as a function of signalsreceived from said transmitter and for transmitting said channelinformation from said receiver to said transmitter.
 24. A receiver asclaimed in claim 22, wherein said detection means is responsive to apermutation signal indicative of the number of permutations over time ofsaid symbols within said block, and which is transmitted from saidtransmitter to said receiver, in the numbers of permutations it performsin detecting the data transmitted.